In a general wireless communication system, only one carrier is considered even though different uplink (UL) and downlink (DL) bandwidths are configured. For example, a wireless communication system in which the number of carriers constituting each of UL and DL is 1 and a UL bandwidth and a DL bandwidth are generally symmetrical may be provided based on a single carrier.
In the International Telecommunication Union (ITU), an IMT-Advanced candidate technique demands support of an extended bandwidth compared with a conventional wireless communication system. However, it is not easy to allocate frequency of a wide bandwidth except in some areas of the world. Accordingly, as a technique for efficiently using a fragmented narrow band, a carrier aggregation (also called bandwidth aggregation or spectrum aggregation) technique generating an effect as if a wide band is logically used by physically grouping a plurality of bands in a frequency domain has been developed.
Carrier aggregation has been introduced to support increased throughput, prevent cost increase caused by introduction of a broadband RF element, and ensure compatibility with a conventional system. Carrier aggregation is a technique for allowing data exchange between a User Equipment (UE) and a Base Station (BS) through a plurality of carrier groups of a bandwidth unit defined in a conventional wireless communication system (an LTE system in the case of an LTE-A system, or an IEEE 802.16e system in the case of an IEEE 802.16m system). Here, a carrier of a bandwidth unit defined in a conventional wireless communication system may be referred to as a Component Carrier (CC). For example, a carrier aggregation technique may include a technique supporting a system bandwidth of up to a maximum of 100 MHz by grouping a maximum of 5 CCs even if one CC supports a bandwidth of 5 MHz, 10 MHz, or 20 MHz.
Meanwhile, as an error control method in UL/DL transmission, an Automatic Repeat and reQuest (ARQ) scheme using error detection information may be used. Moreover, as an improved ARQ scheme, a Hybrid ARQ (HARQ) scheme for performing decoding by combining originally transmitted information with retransmitted information in order to reduce the number of retransmission times caused by occurrence of errors may be used. The HARQ scheme is described in brief by taking UL as an example. A BS receiving UL data on a data channel (PDSCH) from a UE transmits an Acknowledgement/Negative Acknowledgement (ACK/NACK) signal for the UL data on a control channel (PHICH) after a predetermined time period. Although the BS receiving the UL data can transmit the control channel (PHICH) after 4 Transmission Time Intervals (TTIs), the predetermined time is not limited thereto. The ACK/NACK signal is determined as an ACK signal when the UL data is successfully decoded and as a NACK signal when decoding of the UL data fails. If the ACK/NACK signal is determined as a NACK signal, the UE retransmits retransmission data for the UL data to the BS. Retransmission may be performed until the ACK signal is received or up to the maximum number of retransmission times. If the ACK/NACK signal is determined as an ACK signal, the UE may transmit new UL data to the BS. A transmission time point of the ACM/NACK signal or resource allocation for the UL/DL data may be dynamically indicated through signaling by the BS or may be previously scheduled according to the transmission time point of the UL/DL data or resource allocation.
In an LTE system, a TTI of 1 ms is used to reduce a delay time of data transmission and a Round Trip Time (RTT) of HARQ action can be reduced to 8 ms. However, if a UE is located at a cell edge and runs short of power, since sufficient energy cannot be used for data transmission during a short TTI, the probability of successful data transmission may be lowered. Further, if data transmitted to a BS from a UE is important data such as a quality measurement result value of a neighbor cell necessary for performing handover, a problem such as call drop may occur due to failure of data transmission.
Accordingly, a method for increasing the probability of successful data transmission and reception in a cell edge and especially increasing the probability of important data transmission is needed.